Slowing of twitch of dystrophic mouse muscle in partially due to altered activity pattern.

Fast-twitch muscles of the hindlimb of dystrophic (dy2J) mice show a prolongation of both the contraction and relaxation phases of the isometric twitch. Comparable muscles of the forelimb of these mice exhibit relatively little increase in time to peak tension but time to half-relaxation is as severely affected as in the hindlimb. When examined with an immunohistochemical technique to demonstrate the presence of "slow" myosin it was apparent that there were no fibers containing the "slow" isoenzyme in either hindlimb or forelimb muscles of 6-month control mice. In dy2J mice hindlimb muscles contained many fibers with "slow" myosin whereas forelimb muscles did not. It is suggested that the spontaneous twitching activity produced in the hindlimbs, as a result of amyelination of the spinal roots, induces synthesis of "slow" myosin, which in turn leads to prolongation of time to peak twitch tension.